This invention relates to a valve timing control apparatus for use in an internal combustion engine to provide a desired valve timing(s) according to engine operating conditions.
In an article, Motor Fan, Vol. 43, No. 10, pp. 38 and 119, published Oct. 1, 1989, by Saneishobo Inc., there is described a valve timing control apparatus for controlling the intake or exhaust valve timing according to engine operating conditions. The valve timing control apparatus employs a cylindrical gear having internal and external threaded portions one of which is taken in the form of a helical gear. The external threaded portion engages with the inner thread portion of a rotary drum which is formed on its outer peripheral surface with sprocket teeth for engagement with a timing chain engaging with a sprocket mounted on the crankshaft of the engine. The internal threaded portion of the cylindrical gear engages with the external threaded portion formed on the camshaft of the engine. The camshaft is rotated at an angle with respect to the rotary drum by applying fluid pressures to move the cylindrical gear in an axial direction of the camshaft.
However, the conventional valve timing control apparatus employs an expensive helical gear. The helical gear is required to be machined with high accuracy sufficient to provide smooth valve timing control. Additionally, the conventional valve timing control apparatus operates at a slow response rate with respect to engine operating condition changes since the cylindrical gear movement will lag because of the frictional resistances between the cylindrical gear and the internal threaded portion of the rotary drum and between the cylindrical gear and the external threaded portion of the camshaft. Still further, the conventional valve timing control apparatus switches the fluid pressures acting on the front and rear surfaces of the cylindrical gear in an on-off manner to place the camshaft at one of two angular positions with respect to the rotary drum. It is, therefore, difficult, if not impossible, to place the camshaft at a desired angular position between these two angular positions.